Organic light emitting devices having carrier blocking layers comprising metal complexes

ABSTRACT

Light emitting devices having blocking layers comprising one or more metal complexes are provided. The blocking layers may serve to block electrons, holes, and/or excitons. Preferably, the devices further comprise a separate emissive layer in which charge and/or excitons are confined. Metal complexes suitable for blocking layers can be selected by comparison of HOMO and LUMO energy levels of materials comprising adjacent layers in devices of the present invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/226,674, filed Aug. 23, 2002,now U.S. Pat. No. 7,022,421which claims the benefit of U.S. Provisional Patent Application No.60/315,527, filed Aug. 29, 2001, and U.S. Provisional Patent ApplicationNo. 60/317,540, filed Sep. 5, 2001, which are incorporated herein intheir entirety. This application is also related to co-pending U.S.Provisional Application No. 60/317,541, filed Sep. 5, 2001, which isincorporated herein by reference in its entirety

GOVERNMENT RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.F33615-94-1-1414 awarded by DARPA.

RESEARCH AGREEMENTS

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a jointuniversity-corporation research agreement: Princeton University, TheUniversity of Southern California, and the Universal DisplayCorporation. The agreement was in effect on and before the date theclaimed invention was made, and the claimed invention was made as aresult of activities undertaken within the scope of agreement.

FIELD OF THE INVENTION

The present invention is directed to light emitting devicesincorporating metal complexes for improved efficiency and stability.

BACKGROUND OF THE INVENTION

Electronic display currently is a primary means for rapid delivery ofinformation. Television sets, computer monitors, instrument displaypanels, calculators, printers, wireless phones, handheld computers, etc.aptly illustrate the speed, versatility, and interactivity that ischaracteristic of this medium. Of the known electronic displaytechnologies, organic light emitting devices (OLEDs) are of considerableinterest for their potential role in the development of full color,flat-panel display systems that may render obsolete the bulky cathoderay tubes still currently used in many television sets and computermonitors.

Generally, OLEDs are comprised of several organic layers in which atleast one of the layers can be made to electroluminesce by applying avoltage across the device (see, e.g., Tang, et al., Appl. Phys. Lett.1987, 51, 913 and Burroughes, et al., Nature, 1990, 347, 359). When avoltage is applied across a device, the cathode effectively reduces theadjacent organic layers (i.e., injects electrons) whereas the anodeeffectively oxidizes the adjacent organic layers (i.e., injects holes).Holes and electrons migrate across the device toward their respectiveoppositely charged electrodes. When a hole and electron meet on the samemolecule, recombination is said to occur and an exciton is formed.Recombination of the hole and electron is preferably accompanied byradiative emission, thereby producing electroluminescence.

Depending on the spin states of the hole and electron, the exciton whichresults from hole and electron recombination can have either a tripletor singlet spin state. Luminescence from a singlet exciton results influorescence whereas luminescence from a triplet exciton results inphosphorescence. Statistically, for organic materials typically used inOLEDs, about one quarter of the excitons are singlets and the remainingthree quarters are triplets (see, e.g., Baldo, et al., Phys. Rev. B,1999, 60, 14422). Until the discovery that there were certainphosphorescent materials that could be used to fabricate practicalelectro-phosphorescent OLEDs having a theoretical quantum efficiency ofup to 100% (i.e., harvesting all of both triplets and singlets), themost efficient OLEDs were typically based on materials that fluoresced.These materials fluoresced with a maximum theoretical quantum efficiencyof only 25% (where quantum efficiency of an OLED refers to theefficiency with which holes and electrons recombine to produceluminescence), since the triplets to ground state transition is formallya spin forbidden process. Electro-phosphorescent OLEDs have now beenshow to have superior overall device efficiencies as compared withelectro-fluorescent OLEDs (see, e.g., Baldo, et al., Nature, 1998, 395,151 and Baldo, e.g., Appl. Phys. Lett. 1999, 75(3), 4).

Typically, OLEDs contain several thin organic layers between a holeinjecting anode layer, comprising an oxide material, such as indium-tinoxide (ITO), Zn—In—SnO₂, SbO₂, or the like, and an electron injectingcathode layer, comprising a metal layer, such as Mg, Mg:Ag, or LiF:Al.An organic layer residing in proximity to the anode layer is usuallyreferred to as the “hole transporting layer” (HTL) due to its propensityfor conducting positive charge (i.e., holes). Various compounds havebeen used as HTL materials. The most common materials consist of varioustriaryl amines which show high hole mobilities. Similarly, the organiclayer residing in proximity to the cathode layer is referred to as the“electron transporting layer” (ETL) due to its propensity to conductnegative charge (i.e., electrons). There is somewhat more variety in theETL materials used in OLEDs as compared with for the HTL. A common ETLmaterial is aluminum tris(8-hydroxyquinolate) (Alq₃). Collectively, theETL and HTL are often referred to as carrier layers. In some cases, anadditional a layer may be present for enhancing hole or electroninjection from the electrodes into the HTL or ETL, respectively. Theselayers are often referred to as hole injecting layers (HILs) or electroninjecting layer (EIL). The HIL may be comprised of a small molecule,such as 4,4′,4″-tris(30methylphenylphenylamino)triphenylamine (MTDATA)or polymeric material, such as poly(3,4-ethylenedioxythiophene) (PEDOT).The EIL may be comprised of a small molecule material, such as, e.g.,copper phthalocyanine (CuPc). Many OLEDs further comprise an emissivelayer (EL), or alternatively termed, luminescent layer, situated betweenthe ETL and HTL, where electroluminescence occurs. Doping of theluminescent layer with various luminescent materials has allowedfabrication of OLEDs having a wide variety of colors.

In addition to the electrodes, carrier layers, and luminescent layer,OLEDs have also been constructed with one or more blocking layers tohelp maximize efficiency. These layers serve to block the migration ofholes, electrons, and/or excitons from entering inactive regions of thedevice. For example, a blocking layer that confines holes to theluminescent layer effectively increases the probability that holes willresult in a photoemissive event. Hole blocking layers desirably have adeep (i.e., low) HOMO energy level (characteristic of materials that aredifficult to oxidize) and conversely, electron blocking materialsgenerally have a high LUMO energy level. Exciton blocking materials havealso been shown to increase device efficiencies. Triplet excitons, whichare relatively long-lived, are capable of migrating about 1500 to 2000Å, which is sometimes greater than the entire width of the device. Anexciton blocking layer, comprising materials that are characterized by awide band gap, can serve to block loss of excitons to non-emissiveregions of the device.

In seeking greater efficiencies, devices have been experimentallycreated with layers containing light emitting metal complexes.Functional OLEDs having emissive layers oftris(2,2′-bipyridine)ruthenium(II) complexes or polymer derivativesthereof have been reported in Gao, et al., J. Am. Chem. Soc., 2000, 122,7426, Wu, et al., J. Am. Chem. Soc. 1999, 121, 4883, Lyons, et al., J.Am. Chem. Soc. 1998, 120, 12100, Elliot, et al., J. Am. Chem. Soc. 1998,120, 6781, and Maness, et al., J. Am. Chem. Soc. 1997, 119, 3987.Iridium-based and other metal-containing emitters have been reported in,e.g., Baldo, et al., Nature, 1998, 395, 151; Baldo, et al., Appl. Phys.Lett., 1999, 75, 4; Adachi, et al., Appl. Phys. Lett., 2000, 77, 904;Adachi, et al., Appl. Phys. Lett., 2001, 78, 1622; Adachi, et al., Bull.Am. Phys. Soc. 2001, 46, 863, Wang, et al., Appl. Phys. Lett., 2001, 79,449 and U.S. Pat. Nos. 6,030,715; 6,045,930; and 6,048,630. Emissivelayers containing (5-hydroxy)quinoxaline metal complexes as hostmaterial has also been described in U.S. Pat. No. 5,861,219. Efficientmulticolor devices and displays are also described in U.S. Pat. No.5,294,870 and International Application Publication No. WO 98/06242.

A metal-containing blocking layer has also been reported. Specifically,(1,1′-biphenyl)-4-olato)bis(2-methyl-8-quinolinolato N1,O8)aluminum(BAlq) has been used as a blocking layer in the OLEDs reported inWatanabe, et al. “Optimization of driving lifetime durability in organicLED devices using Ir complex,” in Organic Light Emitting Materials andDevices IV, Kafafi, ed. Proceedings of SPIE Vol. 4105, p. 175 (2001).

Although OLEDs promise new technologies in electronic display, theyoften suffer from degradation, short life spans, and loss of efficiencyover time. The organic layers can be irreversibly damaged by sustainedexposure to the high temperatures typically encountered in devices.Multiple oxidation and reduction events can also cause damage to theorganic layers. Consequently, there is a need for the development of newmaterials for the fabrication of OLEDs. Compounds that are stable toboth oxidation and reduction, have high T_(g) values, and readily formglassy thin films are desirable. The invention described hereinbelowhelps fulfill these and other needs.

SUMMARY OF THE INVENTION

The present invention provides, inter alia, light emitting devicescomprising at least one blocking layer, wherein the blocking layerincludes at least one transition metal complex.

In some embodiments, the blocking layer does not electroluminescence insaid device. In other embodiments, the blocking layer is a hole blockinglayer, an electron blocking layer, or an exciton blocking layer. Theblocking layer can consist essentially of said transition metal complex.Further, the transition metal can be a second or third row transitionmetal, such as iridium. The metal complex can also bebis(2-(4,6-difluorophenyl)pyridyl-N, C2 ′)iridium(III) picolinate.

The present invention further provides light emitting devices comprisingat least one blocking layer, wherein the blocking layer comprises atleast one metal complex comprising a main group metal atom having anatomic number greater than 13. In some embodiments, the blocking layercan be hole blocking layer, an electron blocking layer, or an excitonblocking layer. In some embodiments, the blocking layer consistsessentially of said metal complex. The main group metal atom can be athird, fourth, or fifth main group metal atom, such as gallium. Further,the metal complex can begallium(III)tris[2-(((pyrrole-2-yl)methylidene)amino)ethyl]amine.

According to other aspects, the present invention provides lightemitting devices comprising at least one blocking layer, wherein theblocking layer includes at least one metal complex comprising a maingroup metal atom, wherein the complex is six-coordinate.

In further aspects, the present invention provides light emittingdevices comprising at least one blocking layer, wherein the blockinglayer includes at least one metal complex, and wherein the metal complexcomprises aluminum and is not BAlq.

Also provided by the present invention are light emitting devicescomprising a blocking layer, wherein the blocking layer includes a wideband-gap organic matrix into which a metal complex is doped. In someembodiments, the wide band-gap organic matrix is doped with about 1 toabout 50% by weight of metal complex. In further embodiments, theorganic matrix can comprise octaphenyl cyclooctatetraene or oligophenyl.

In still further embodiments, the present invention provides lightemitting devices comprising at least one blocking layer comprising acompound of formula III:

wherein:

-   -   M is a metal atom;    -   X is N or CX′ where X′ is H, C₁-C₂₀ alkyl, C₂-C₄₀ mono- or poly        alkenyl, C₂-C₄₀ mono- or poly alkynyl, C₃-C₈ cycloalkyl, aryl,        heteroaryl, aralkyl, heteroaralkyl, or halo;    -   A is CH, CX′, N, P, P(═O), aryl or heteroaryl;    -   each R¹ and R² is, independently, H, C₁-C₂₀ alkyl, C₂-C₄₀        alkenyl, C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl, aryl, aralkyl, or        halo; or    -   R¹ and R², together with the carbon atoms to which they are        attached, link to form a fused C₃-C₈ cycloalkyl or aryl group;

R³ is H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl, C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl,aryl aralkyl, or halo; and

-   -   n is 1 to 5. In some embodiments, M is Al or Ga. In further        embodiments, R¹ and R² are linked to form a fused phenyl group.        In further embodiments, A is N.

In further aspects, the invention provides light emitting devicescomprising an emissive layer and a hole blocking layer, each of saidlayers having an anode side and a cathode side, wherein said cathodeside of said emissive layer is in contact with said anode side of saidhole blocking layer, wherein said hole blocking layer has a lower HOMOenergy level than the HOMO energy level of said emissive layer andcomprises at least one transition metal complex. In some embodiments,the magnitude of the difference between the LUMO energy levels of thehole blocking layer and the emissive layer is less than the magnitude ofthe difference between the HOMO energy levels of the hole blocking layerand the emissive layer. In further embodiments, the hole blocking layerconsists essentially of said metal complex. In further embodiments, theemissive layer comprises a host material doped with an emitter. In yetfurther embodiments, the hole blocking layer comprises a wide band-gaporganic matrix doped with said metal complex. In some embodiments, thedoped metal complex has a smaller band-gap than the matrix. In furtherembodiments, the LUMO energy level of the doped metal complex is lessthan about 200 meV from the LUMO energy level of the emissive layer. Infurther aspects, the emitter is a metal complex.

The present invention further provides light emitting devices comprisingan emissive layer and a hole blocking layer, each of said layers havingan anode side and a cathode side, wherein said cathode side of saidemissive layer is in contact with said anode side of said hole blockinglayer, wherein said hole blocking layer has a lower HOMO energy levelthan the HOMO energy level of said emissive layer and comprises at leastone metal complex comprising a main group metal atom having an atomicnumber greater than 13.

The present invention further provides light emitting devices comprisingan emissive layer and a hole blocking layer, each of said layers havingan anode side and a cathode side, wherein said cathode side of saidemissive layer is in contact with said anode side of said hole blockinglayer, wherein said hole blocking layer has a lower HOMO energy levelthan the HOMO energy level of said emissive layer and comprises at leastone six-coordinate metal complex.

The present invention further provides light emitting devicse comprisingan emissive layer and a hole blocking layer, each of said layers havingan anode side and a cathode side, wherein said cathode side of saidemissive layer is in contact with said anode side of said hole blockinglayer, wherein said hole blocking layer has a lower HOMO energy levelthan the HOMO energy level of said emissive layer and comprises at leastone metal complex comprising aluminum, wherein said metal complex is notBalq.

The present invention further provides light emitting devices comprisingan emissive layer and an electron blocking layer, each of said layershaving an anode side and a cathode side, wherein said anode side of saidemissive layer is in contact with said cathode side of said electronblocking layer, wherein said electron blocking layer has a higher LUMOenergy level than the LUMO energy level of said emissive layer andcomprises at least one metal complex. In some embodiments, the magnitudeof the difference between the HOMO energy levels of the electronblocking layer and the emissive layer is less than the magnitude of thedifference between the LUMO energy levels of said hole blocking layerand the emissive layer. In other embodiments, the electron blockinglayer has a HOMO energy level that is less than about 200 meV from theHOMO energy level of the emissive layer. In yet further embodiments, theelectron blocking layer consists essentially of said metal complex. Inother embodiments, the emissive layer comprises a host material dopedwith an emitter. In yet further embodiments, the electron blocking layercomprises a wide-band gap organic matrix doped with the metal complex.In some embodiments, the doped metal complex has a smaller band-gap thanthe matrix. In further embodiments, the HOMO energy level of the dopedmetal complex is less than about 200 meV from the HOMO energy level ofthe emissive layer.

In yet other embodiments, the present invention provides light emittingdevices comprising an emissive layer and an exciton blocking layer,wherein the emissive layer is in contact with the exciton blockinglayer, wherein the exciton blocking layer has a wider optical gap thanthe optical gap of the emissive layer, and wherein the exciton blockinglayer comprises at least one metal complex. In some embodiments, theexciton blocking layer has a HOMO energy level that is less than about200 meV from the HOMO energy level of said emissive layer. In furtherembodiment, the exciton blocking layer has a LUMO energy level that isless than about 200 meV from the LUMO energy level of said emissivelayer. In other embodiments, said exciton blocking layer consistsessentially of said metal complex.

The present invention further provides light emitting devices having thestructure anode/HTL/EL/HBL/ETL/cathode wherein the HBL comprises a wideband-gap organic matrix doped with a metal complex.

The present invention further provides light emitting devices having thestructure anode/HTL/EBL/EL/ETL/cathode wherein the EBL comprises a wideband-gap organic matrix doped with a metal complex.

The present invention further provides methods of confining holes to anemissive layer in a light emitting device, wherein the emissive layercomprises an anode side and a cathode side, and wherein the devicecomprises a blocking layer adjacent to the cathode side of said emissivelayer, where the blocking layer has a lower HOMO energy level than theHOMO energy level of the emissive layer and comprises at least onetransition metal complex.

In further aspects, the invention provides methods of confining holes toan emissive layer in a light emitting device, wherein the emissive layercomprises an anode side and a cathode side, and wherein the devicecomprises a blocking layer adjacent to the cathode side of the emissivelayer, where the blocking layer has a lower HOMO energy level than theHOMO energy level of the emissive layer and comprises at least one metalcomplex comprising a main group metal atom having an atomic numbergreater than 13.

In still further aspects, the invention provides methods of confiningholes to an emissive layer in a light emitting device, wherein theemissive layer comprises an anode side and a cathode side, and whereinthe device comprises a blocking layer adjacent to said cathode side ofsaid emissive layer, where the blocking layer has a lower HOMO energylevel than the HOMO energy level of the emissive layer and comprises atleast one six-coordinate metal complex.

In still further aspects, the invention provides methods of confiningholes to an emissive layer in a light emitting device, wherein theemissive layer comprises an anode side and a cathode side, and whereinthe device comprises a blocking layer adjacent to the cathode side ofsaid emissive layer, where the blocking layer has a lower HOMO energylevel than the HOMO energy level of the emissive layer and comprises atleast one metal complex comprising aluminum, wherein the metal complexis not BAlq.

In still further aspects, the invention provides methods of confiningelectrons to an emissive layer in a light emitting device, wherein theemissive layer comprises an anode side and a cathode side, and whereinthe device comprises a blocking layer adjacent to the cathode side ofthe emissive layer, where the blocking layer has a higher LUMO energylevel than the LUMO energy level of the emissive layer and comprises atleast one metal complex. The method preferably comprises applying avoltage across such a device.

The present invention further provides methods of confining excitons toan emissive layer in a light emitting device, wherein the emissive layeris in contact with the exciton blocking layer, wherein the excitonblocking layer has a wider optical gap than the optical gap of theemissive layer, and wherein the exciton blocking layer comprises atleast one metal complex, the methods comprising applying a voltageacross the device.

In further aspects, the invention provides methods of fabricating alight emitting device, the methods comprising depositing a blockinglayer onto a preexisting layer wherein the blocking layer comprises acompound of formula III as described above. In some embodiments, thecompound is Ga(pma)₃.

In yet another aspect, the present invention provides methods offabricating a light emitting device, the methods comprising depositing ablocking layer onto a preexisting layer, wherein the blocking layercomprises a metal complex comprising iridium. In some embodiments, themetal complex is FIrpic.

In some embodiments of the foregoing devices and methods, the blockinglayer consists essentially of the metal complex. In further embodimentsof the foregoing devices and methods, the blocking layer comprises anorganic matrix doped with the metal complex, preferably wherein theorganic matrix is a wide band-gap organic matrix.

In further aspects, the invention provides pixels and displayscomprising the light emitting devices described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 portrays several iridium compounds suitable for preparing devicesof the present invention.

FIG. 2 shows current v. voltage plots for devices of the structureITO/NPD(400 Å)/CBP:Irppy₃(6% 200 Å)(facial)/HBL(150 Å)/Alq₃(200Å)/Mg:Ag.

FIGS. 3A-3B show quantum efficiency v. current density and normalized ELv. wavelength plots for devices of the structure ITO/NPD(400Å)/CBP:Irppy₃(6% 200 Å)(facial)/HBL(150 Å)/Alq₃(200 Å)/Mg:Ag.

FIG. 4 shows quantum efficiency v. current density (lower) and lumens v.current density (upper) plots for a device having a FIrpic blockinglayer.

FIG. 5 shows quantum efficiency v. current density (lower) and lumens v.current density (upper) plots for a device having a FIrpic blockinglayer.

FIG. 6 shows current density v. voltage plots for devices having aFIrpic blocking layer.

FIG. 7 shows the electroluminescence spectrum for devices having aFIrpic blocking layer.

FIGS. 8A-8D show plots comparing properties of devices of the structureNPD(400 Å)/CBP:FIrpic(6%)(300 Å)/HBL(200 Å)/Alq₃(200 Å)/Mg:Ag.

FIG. 9 compares properties of devices of the structureNPD/CBP:FIrpic/HBL/Alq₃.

FIGS. 10A-10B show electronic spectra for Ga(pma)₃.

FIG. 11 shows a current density v. voltage plot for a device having thestructure ITO/Co(ppz)₃(400 Å)/Ga(Pma)₃(100 Å)/Alq₃(500 Å)/MgAg(1000Å)/Ag.

FIG. 12 shows a luminance v. voltage plot for devices of the structureITO/Co(Ppz)₃(400 Å)/Ga(pma)₃(100 Å)/Alq₃(500 Å)/MgAg(1000 Å)/Ag.

FIG. 13 shows an external quantum efficiency v. voltage plot for devicesof the structure ITO/Co(Ppz)₃(400 Å)/Ga(pma)₃(100 Å)/Alq₃(500Å)/MgAg(1000 Å)/Ag.

FIG. 14 shows a quantum efficiency v. current density plot for deviceshaving the structure ITO/Co(Ppz)₃(400 Å)/Ga(pma)₃(100 Å)/Alq₃(500Å)/MgAg(1000 Å)/Ag.

FIGS. 15A-15B show current density v. voltage and brightness v. voltageplots for devices having the structure ITO/HTL(500 Å)/CBP:Irppy(6%)(200Å)/BCP(150 Å)Alq₃(200 Å)/LiF/Al.

FIGS. 16A-16B show quantum efficiency v. current density and emissionspectrum plots for devices having the structure ITO/HTL(500Å)/CBP:Irppy(6%)(200 Å)/BCP(150 Å)Alq₃(200 Å)/LiF/Al.

FIG. 17 illustrates benefits of hole blocking layers in light emittingdevices.

FIG. 18 further illustrates benefits of hole blocking layers in lightemitting devices.

FIG. 19 shows conventional hole blocking layer materials used in lightemitting devices.

FIG. 20 illustrate the advantages of metal complexes as hole blockinglayer material.

FIG. 21 illustrates a host-dopant approach to constructing hole blockinglayers.

FIG. 22 illustrates devices having a hole blocking layer with ahost-dopant structure.

FIG. 23 shows materials for hosts and dopants in hole blocking layers.

FIG. 24 compares properties of devices having different hole blockinglayers comprising FIrpic.

FIG. 25 compares properties of different devices having an emissivelayer and a hole blocking layer both comprising FIrpic.

FIG. 26 compares properties of devices having different HBLs.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As used herein, the terms “low” and “deep,” in reference to molecularorbital energies, are used interchangeably. Lower and deeper generallydescribe molecular orbitals residing at a lower, or more stable, energylevel. Ionization of electrons from deeper orbitals requires more energythan ionization of electrons in shallower, or higher, orbitals. Thus,although the deeper orbitals are said to be lower, they are oftenreferred to numerically by higher numbers. For example, a molecularorbital residing at 5.5 eV is lower (deeper) than a molecular orbitalresiding at 2.5 eV. Similarly, the terms “shallow” and “high” inreference to orbital energy levels, refer to orbitals at less stableenergies. These terms are well known to those skilled in the art.

As used herein, the term “adjacent,” in reference to layers of lightemitting devices, refers to layers having contacting sides. For example,a first layer and a second layer that are adjacent to one anotherdescribe, for example, contacting layers where one side of one layer isin contact with one side of the other layer.

As used herein, the term “gap” or “band-gap” generally refers to anenergy difference, such as, for example, between a HOMO and a LUMO. A“wider gap” refers to an energy difference that is greater than for a“narrower gap” or “smaller gap.” A “carrier gap” refers to the energydifference between the HOMO and LUMO of a carrier.

The present invention is directed to, inter alia, light emitting devicescomprising one or more layers that in turn comprise at least one metalcomplex. Devices, as such, may have higher efficiencies and higherstability as compared with devices having traditional organic blockinglayers.

The light emitting devices of the present invention are typicallylayered structures that electroluminesce when a voltage is appliedacross the device. Typical devices are structured so that one or morelayers are sandwiched between a hole injecting anode layer and anelectron injecting cathode layer. The sandwiched layers have two sides,one facing the anode and the other facing the cathode. These sides arereferred to as the anode side and the cathode side, respectively. Layersare generally deposited on a substrate, such as glass, on which eitherthe anode layer or the cathode layer may reside. In some embodiments,the anode layer is in contact with the substrate. In many cases, forexample, when the substrate comprises a conductive or semi-conductivematerial, an insulating material can be inserted between the electrodelayer and the substrate. Typical substrate materials, that may be rigid,flexible, transparent, or opaque, include glass, polymers, quartz,sapphire, and the like.

Hole transporting layers are placed adjacent to the anode layer tofacilitate the transport of holes. In some embodiments, a hole injectinglayer for enhancing hole injection, sometimes referred to as a holeinjecting enhancement layer, may be placed adjacent to the anode,between the anode and the HTL. Materials suitable for the HTL includeany material that is known by one skilled in the art to function assuch. Suitable materials are typically easy to oxidize and includetriaryl amines, such asN,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′biphenyl-4,4′diamine (TPD),4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD),4,4′-bis[N-(2-naphthyl)-N-phenyl-amino]biphenyl (β-NPD), and the like.Metal complexes may also be used in HTLs. Some suitable metal complexesare described, for example, in Application Ser. No. 60/283,814, filedApr. 13, 2001, which is incorporated herein by reference in itsentirety. Similarly, ETLs are situated adjacent to the cathode layer tofacilitate transport of electrons. An electron injecting enhancementlayer can optionally be placed adjacent to an ETL or cathode layer.Materials suitable for the ETL include any materials known in the art tofunction as such. Typical ETL materials are relatively easy to reduceand can include, for example, aluminum tris(8-hydroxyquinolate) (Alq₃),carbazoles, oxadiazoles, triazoles, thiophene, oligothiophene, and thelike. HTL and ETL carrier layers can have thicknesses ranging from about100 to about 1000 Å. Since it is typically the site of exciton formationand luminescence, the EL layer is preferably somewhere between the HTLand ETL. The EL can optionally be in contact with one or both of the HTLand ETL or may be flanked by one or more blocking layers. EL materialscan include, for example dye-doped Alq₃ and the like. In someembodiments, neat (un-doped) films of luminescent material may be usedas the emissive layer. Furthermore, layers can serve dual functions. Forexample, an ETL or HTL can also function as an EL.

In some embodiments, it is desirable that one or more layers of thedevice comprise one or more dopants. Emissive dopants (i.e.,photoemitting molecules, emitters) can be included in at least onelayer, such as for example the EL, for improved efficiency and colortunability. Doped layers usually comprise a majority of host materialand minority of dopant. Host material (also referred to as matrix)typically transfers excitons through a non-radiative process to theemissive dopant material, which then emits light of a wavelengthcharacteristic of the dopant, rather than the host.

Dopants can also serve to trap charge. For example, the LUMO levels ofthe host and dopant can be arranged such that the LUMO level of thedopant is lower than the LUMO level of the host, such that the dopantmolecule can act as an electron trap. Similarly, the HOMO levels of thehost and dopant can be arranged such that the HOMO level of the dopantis higher than the HOMO level of the host, such that the dopant moleculewould act as a hole trap. In addition, one or more dopants, referred toas transfer dopants, can be used to facilitate the transfer of energyfrom the host to the emissive dopant. For example, cascade doping can beused, which involves the non-radiative transfer of excitons from amolecule of the host through one or more transfer dopants to theemissive dopant. These intermediate transfers can be by Förstertransfer, Dexter transfer, hole trapping or electron trapping thateventually leads to the formation of an exciton on the transfer dopantor the emissive dopant, or by any other suitable mechanism.

Dopants can be present in the host material in quantities ranging, forexample, from about 0.1% to about 50%, from about 1% to about 20%, orfrom 1% to about 10% by weight. A level of about 1% by weight of dopingis preferred for emissive dopants in host material. Alternatively, insome embodiments, levels of dopant result in an average intermoleculardistance between dopant molecules of about the Förster radius of thedopant, such as, for example, from about 20 to about 40 Å, or from about25 to about 35 Å, or about 30 Å. Emissive dopants can include anycompound that is capable of photoemission. Emissive dopants includefluorescent organic dyes, such as laser dyes, as known and used in theart. Preferred emissive dopants include phosphorescent metal complexes,such as the Ir, Pt, and other heavy metal complexes disclosed in U.S.Pat. Nos. 6,303,238, 6,830,828, and U.S. Provisional Patent ApplicationNo. 60/283,814, filed Apr. 13, 2001, each of which is incorporatedherein by reference in its entirety.

In some embodiments, devices of the present invention comprise at leastone blocking layer. Blocking layers (BLs) function to confine holes,electrons, and/or excitons to specific regions of the light emittingdevices. For example, device efficiency can be increased when excitonsare confined to the EL and/or when holes and electrons are preventedfrom migrating out of the EL. Blocking layers can serve one or moreblocking functions. For example, a hole blocking layer can also serve asan exciton blocking layer. In some embodiments, the hole blocking layerdoes not simultaneously serve as an emissive layer in devices of thepresent invention. Although a blocking layer can include compounds thatare capable of emitting, emission can occur in a separate emissivelayer. Thus, in preferred embodiments, the blocking layer does notluminesce. Blocking layers can be thinner than carrier layers. Typicalblocking layers have thicknesses ranging from about 50 Å to about 1000Å, or from about 50 Å to about 750 Å, or from about 50 Å to about 500 Å.Additionally, blocking layers preferably comprise compounds other thanBAlq.

Hole blocking layers (HBLs) are typically comprised of materials thathave difficulty acquiring a hole. For example, hole blocking materialscan be relatively difficult to oxidize. In most instances, hole blockingmaterials are more difficult to oxidize than an adjacent layer fortransporting holes. A material that is more difficult to oxidize thananother material typically possesses a lower HOMO energy level. Forexample, holes originating from the anode and migrating into an EL canbe effectively blocked from exiting the EL (on the cathode side) byplacing a blocking layer of material adjacent to the EL on the cathodeside of the device. The blocking layer preferably has a HOMO energylevel lower than the HOMO energy levels of the EL. Larger differences inHOMO energy levels correspond to better hole blocking ability. The HOMOof the materials of the blocking layer are preferably at least about 50,100, 200, 300, 400, 500 meV (milli-electronvolts) or more deeper thanthe HOMO level of an adjacent layer in which holes are to be confined.In some embodiments, the HOMO of the materials of the blocking layer areat least about 200 meV deeper than the HOMO level of an adjacent layerin which holes are to be confined.

In some devices of the invention, the layer in which holes are to beconfined can comprise more than one material, such as a host material(matrix) and a dopant. In this case, a HBL preferably has a HOMO energylevel that is lower (deeper) than the material of the adjacent layerwhich carries the majority of positive charge (i.e., the material withthe highest (shallowest) HOMO energy level). For example, an emissivelayer can comprise a host material having a deeper HOMO energy levelthan the dopant. In this case, the dopant acts as a trap for holes andcan be the principle hole transporter of the emissive layer. Thus, insuch embodiments, the HOMO energy of the dopant is considered whenselecting a hole blocking layer. Thus, in some embodiments, the HOMOenergy level of the HBL can be higher than the host material and lowerthan that of the dopant.

Hole blocking layers are also preferably good electron injectors.Accordingly, the LUMO energy level of the HBL is preferably close to theLUMO energy level of the layer in which holes are to be confined.Differences in LUMO energy levels between the two layers in someembodiments can be less than about 500 meV, 200 meV, 100 meV, 50 meV, oreven smaller. Hole blocking layers that are also good electron injectorstypically have smaller energy barriers to electron injection than forhole leakage. Accordingly, the difference between the LUMO energies ofthe HBL and the layer in which holes are to be confined (correspondingto an electron injection energy barrier) is smaller than the differencein their HOMO energies (i.e., hole blocking energy barrier).

Conversely, electron blocking layers (EBLs) are comprised of materialsthat have difficulty acquiring electrons (i.e., are relatively difficultto reduce). In the context of a light emitting device, EBLs arepreferably more difficult to reduce than the adjacent layer from whichelectrons migrate. A material that is more difficult to reduce thananother material generally has a higher LUMO energy level. As anexample, electrons originating from the cathode and migrating into an ELlayer can be blocked from exiting the EL (on the anode side) by placinga blocking layer adjacent to the anode side of the EL where the blockinglayer has a LUMO energy level higher than the LUMO energy level of theEL. Larger differences in LUMO energy levels correspond to betterelectron blocking ability. The LUMO of the materials of the blockinglayer are preferably at least about 50 meV, 100 meV, 200 meV, 300 meV,400 meV, 500 meV or more higher (shallower) than the LUMO level of anadjacent layer in which holes are to be confined. In some embodiments,the LUMO of the materials of the blocking layer can be at least about200 meV higher (shallower) than the LUMO level of an adjacent layer inwhich holes are to be confined.

In some embodiments, the layer in which electrons are to be confined cancomprise more than one material, such as a host material (matrix) and adopant. In this case, an EBL preferably has a LUMO energy level that ishigher than the material of the adjacent layer which carries themajority of negative charge (e.g., either the host or dopant having thelowest LUMO energy level). For example, an emissive layer can include ahost material having a deeper LUMO energy level than the dopant. In thiscase, the host can be the principle electron transporter of the emissivelayer. In such embodiments, the LUMO energy level of the EBL can behigher than the host material and lower than that of the dopant.Similarly, if the dopant served as the primary carrier of electrons,then the EBL preferably has a higher LUMO than the dopant.

Electron blocking layers are also preferably good hole injectors.Accordingly, the HOMO energy level of the EBL is preferably close to theHOMO energy level of the layer in which electrons are to be confined.Differences in HOMO energy levels between the two layers in someembodiments can be less than about 500 meV, 200 meV, 100 meV, 50 meV, oreven smaller. Electron blocking layers that are also good hole injectorstypically have smaller energy barriers to hole injection than forelectron leakage. Accordingly, the difference between the HOMO energiesof the EBL and the layer in which electrons are to be confined(corresponding to an hole injection energy barrier) is smaller than thedifference in their LUMO energies (i.e., electron blocking energybarrier).

Migration of excitons from the EL to other parts of the devices can beblocked with materials that have difficulty acquiring excitons. Transferof an exciton from one material to another may be prevented when thereceiving material has a wider (greater) optical gap than the excitondonating material. For example, excitons can be substantially confinedto the EL layer of a device by placing, adjacent to the EL layer, anexciton blocking layer having a wider optical gap than the materialscomprising the EL layer. Exciton blocking layers can also be placed oneither side of the EL. Exciton blocking layers can also serve as HBLs orEBLs, depending on the energy levels of the HOMO or LUMO of the excitonblocking material compared with those of adjacent layers (as discussedabove). Additionally, exciton blocking layers can be good electron orhole injectors when either the HOMO or LUMO energy level of the excitonblocking layer is close in energy to the respective HOMO or LUMO energylevel of an adjacent layer. For example, in devices having an excitonblocking layer and an emissive layer, the exciton blocking layer canhave a HOMO energy level that is less than about 500, 200, or 100 meVfrom the HOMO energy level of said emissive layer. Conversely, theexciton blocking layer can have a LUMO energy level that is less thanabout 500, 200, 100 meV from the LUMO energy level of said emissivelayer.

According to some embodiments of the present invention, blocking layerscan also comprise dopants. As an example, the blocking layer can becomprised of a wide band-gap matrix (host) material doped with a smallerband-gap dopant. Depending on the matrix and dopant combination, theeffective LUMO energy of the blocking layer can be lowered by thepresence of dopant, consequently improving the electron conduction andinjection properties of a hole blocking layer. Conversely, the effectiveHOMO energy of the blocking layer can be raised by the presence ofdopant, thereby improving hole injection properties. As an example, insome embodiments, HBLs comprise a wide band-gap matrix doped with asmaller band-gap material where the deep HOMO energy level of the matrixserves to prevent transport of holes and the relatively shallow LUMOlevel of the dopant favors electron injection. In some embodiments ofthe invention, the matrix can comprise a substantially conjugatedorganic molecule, such as, for example, octaphenyl cyclooctatetraene(OPCOT), oligophenylenes, such as hexaphenyl, and other similarmaterials having a wide band-gap. Suitable matrix band gap values can beat least about 3 eV, but can also be at least about 2.5 eV, 3.0 eV, 3.3eV, 3.5 eV or higher. Dopant is preferably a metal complex. Dopinglevels can range from about 1% to about 50%, or more preferably fromabout 5% to about 20%, or even more preferably from about 10 to about15% by weight. An example of a suitable metal complex used as a dopantfor blocking layers is bis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III) picolinate (FIrpic). An example of hole blocking layercomprising a matrix doped with a metal complex is OPCOT doped with 15%by weight of FIrpic (OPCOT:FIrpic(15%)). For example, OPCOT:FIrpic caneffectively confine holes to an emissive layer comprising CBP doped withIr(ppy)₃ (tris(2-phenylpyridyl-N, C2′)iridium(III), Irppy) because theHOMO of OPCOT is lower than the HOMO of Irppy and the LUMO of FIrpic ishigher than the LUMO of CBP (see, e.g., Examples 7 and 8).

Metal complexes used in the devices of the present invention include anymetal coordination complex comprising at least one metal atom and atleast one ligand. Metal complexes can be charged or uncharged; however,uncharged complexes are more amenable to the thin layer depositiontechniques used in OLED fabrication. Metal complexes are preferablystable to both one electron oxidation and one electron reductionprocesses. Redox-stable complexes can be identified, for example, bycyclic voltammetry (e.g., identification of reversible redox events).Additionally, such metal complexes often have low reorganizationalenergy barriers associated with oxidation and reduction. Accordingly,complexes having low reorganizational energy barriers show littlestructural difference between resting state, oxidized, and reducedstate. Metal complexes typically characterized as having lowreorganizational energy barriers include complexes having d⁰, d¹, d²,d³, d⁴, d⁵ and d⁶ electron configurations. For example, octahedralcomplexes having d³ or d⁶ metals typically generally have lowreorganizational energy barriers. Metal complexes in which redox eventsaffect predominantly non-bonding molecular orbitals (such as the t_(2g)set in octahedral transition metal complexes) generally have lowreorganizational energy barriers, since little structural change is seenin the ligand set upon oxidation or reduction. Reorganizational energyassociated with redox events can also be modulated by the ligand set.For example, multidentate ligands can structurally impose a certaincoordination geometry in metal complexes. Relatively rigid tridentate,tetradentate, hexadentate ligands, and the like can constraincoordination geometry such that redox events do not result insignificant structural reorganization. Additionally, metal complexesthat are coordinatively saturated, such as six-coordinate complexes,which are less likely to have significant structural change associatedwith oxidation or reduction, are also preferred. Four-coordinatecomplexes can also be suitable and can include both tetrahedral andsquare-planar complexes as well as others. Octahedral complexes are alsosuitable due to their propensity for forming glassy films. Metalcomplexes comprising aromatic ligands may help facilitate redoxprocesses, preferably in those instances where redox events are largelycentered on the ligand. Furthermore, metal complexes comprising heavymetals are preferred over those with lighter metals for their greaterthermal stability. For example, complexes comprising second and thirdrow transition metals are preferred.

Any metal atom, in any of its accessible oxidation states, is suitablein metal complexes, including main group, transition metals,lanthanides, actinides, alkaline earth, and alkali metals. Transitionmetals include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc,Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg. Main groupmetals include Al, Ga, Ge, In, Sn, Sb, Tl, Pb, Bi, and Po. In someembodiments, metals having an atomic number greater than about 13, 36,or 54 are preferred.

Metal complexes can include any suitable ligand system. Suitable ligandscan be monodentate, bidentate, multidentate, π-bonding, organic,inorganic, charged, or uncharged. Further, ligands preferably compriseone or more heteroatoms through which the metal atom is coordinated,although organometallic compounds comprising coordinating carbon arealso suitable and also considered as metal complexes. Coordinatingheteroatoms of the ligands can include oxygen, nitrogen, sulphur,phosphorus, and the like. Nitrogen-containing ligands can includeamines, nitrenes, azide, diazenes, triazenes, nitric oxide,polypyrazolylborates, heterocycles, such as 2,2′-bipyridine (bpy),1,10-phenanthroline, terpyridine (trpy), pyridazine, pyrimidine, purine,pyrazine, pyridine, 1,8-napthyridine, pyrazolate, imidazolate, andmacrocycles including those with and without a conjugated π system, andthe like. Phosphorus-containing ligands typically include phosphines andthe like. Oxygen-containing ligands include water, hydroxide, oxo,superoxide, peroxide, alkoxides, alcohols, aryloxides, ethers, ketones,esters, carboxylates, crown ethers, β-diketones, carbamate,dimethylsulfoxide, and oxo anions, such as carbonate, nitrate, nitrite,sulfate, sulfite, phosphate, perchlorate, molybdate, tungstate, oxalateand related groups. Sulfur-containing ligands can include hydrogensulfide, thiols, thiolates, sulfides, disulfides, thioether, sulfuroxides, dithiocarbamates, 1,2-dithiolenes, and the like. Ligandscomprising coordinating carbon atoms can include cyanide, carbondisulfide, alkyl, alkenes, alkynes, carbide, cyclopentadienide, and thelike. Halides can also serve as ligands. Metal complexes containingthese and other ligands are described in detail in Cotton and Wilkinson,Advanced Inorganic Chemistry, Fourth Ed., John Wiley & Sons, New York,1980, which is incorporated herein by reference in its entirety.Additional suitable ligands are described in U.S. ProvisionalApplication No. 60/283,814, filed Apr. 13, 2001 and U.S. Pat. No.6,830,828, each of which is incorporated herein by reference in itsentirety.

Ligands, especially neutral ligands, can be further derivatized with oneor more substituents, including anionic groups, to fully or partiallyneutralize any positive formal charge associated with the metal atoms ofthe metal complexes. Suitable anionic substitutents can includecarbonate, nitrate, nitrite, sulfate, sulfite, phosphate, and the like.

Metal complexes suitable for hole blocking layers can include, interalia, complexes of Os, Ir, Pt, and Au, including those described inapplication Ser. No. 08/980,986, filed Dec. 1, 1997; U.S. Pat. Nos.6,303,238, 09/883,734, filed Jun. 18, 2001; U.S. Pat. Nos. 6,830,828;and 60/283,814, filed Apr. 13, 2001, each of which is hereinincorporated by reference in its entirety. An example of a metalcomplex. suitable in hole blocking layers isbis(2-(4,6-difluorophenyl)pyridyl-N, C2′)iridium(III) picolinate(FIrpic), the structure of which is shown below.

bis(2-(4,6-difluorophenyl)pyridyl-N, C2′)iridium(III) picolinate(FIrpic)

Metal complexes suitable for electron blocking layers include those thatare relatively difficult to reduce (i.e., high LUMO energy level).Suitable metal complexes include metal complexes of the formula:

-   -   M is a metal atom;    -   X is N or CX′ where X′ is H, C₁-C₂₀ alkyl, C₂-C₄₀ mono- or poly        alkenyl, C₂-C₄₀ mono- or poly alkynyl, C₃-C₈ cycloalkyl, aryl,        heteroaryl, aralkyl, heteroaralkyl, or halo;    -   A is CH, CX′, N, P, P(═O), aryl or heteroaryl;    -   each R¹ and R² is, independently, H, C₁-C₂₀ alkyl, C₂-C₄₀        alkenyl, C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl, aryl, aralkyl, or        halo; or    -   R¹ and R², together with the carbon atoms to which they are        attached, link to form a fused C₃-C₈ cycloalkyl or aryl group;    -   R³ is H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl, C₂-C₄₀ alkynyl, C₃-C₈        cycloalkyl, aryl, aralkyl, or halo; and    -   n is 1 to 5.

In some preferred embodiments, M is a trivalent metal, such as Al or Ga.Variable A may preferably be CR³ or N. R¹ and R², in some embodiments,join to form a fused aromatic ring, such as phenyl or pyridyl. Aparticularly suitable compound of the above formula isgallium(III)tris[2-(((pyrrole-2-yl)methylidene)amino)ethyl]amine(Ga(pma)₃) shown below.

Other suitable metal complexes may have the formula

wherein:

-   -   M is a metal atom;    -   is N or CX′ where X′ is H, C₁-C₂₀ alkyl, C₂-C₄₀ mono- or poly        alkenyl, C₂-C₄₀ mono- or poly alkynyl, C₃-C₈ cycloalkyl, aryl,        heteroaryl, aralkyl, heteroaralkyl, or halo;    -   each R¹ and R² is, independently, H, C₁-C₂₀ alkyl, C₂-C₄₀        alkenyl, C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl, aryl, aralkyl, or        halo; or    -   R¹ and R², together with the carbon atoms to which they are        attached, link to form a fused C₃-C₈ cycloalkyl or aryl group;        and    -   R³ is H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl, C₂-C₄₀ alkynyl, C₃-C₈        cycloalkyl, aryl, aralkyl, or halo.

As referred to throughout the present disclosure, alkyl groups includeoptionally substituted linear and branched aliphatic groups. Cycloalkylrefers to cyclic alkyl groups, including, for example, cyclohexyl andcyclopentyl, as well as heterocycloalkyl groups, such as pyranyl, andfuranyl groups. Cycloalkyl groups may be optionally substituted. Alkenylgroups may be substituted or unsubstituted and comprise at least onecarbon-carbon double bond. Alkynyl groups may be substituted orunsubstituted and comprise at least one carbon-carbon triple bond. Arylgroups are aromatic and substituted aromatic groups having about 3 toabout 50 carbon atoms, including, for example, phenyl. Heteroaryl groupsare aromatic or substituted aromatic groups having from about 3 to about50 carbon atoms and comprising at least one heteroatom. Examples ofheteroaryl groups include pyridyl and imidazolyl groups. Aralkyl groupscan be substituted or unsubstituted and have about 3 to about 30 carbonatoms, and include, for example, benzyl. Heteroaralkyl include aralkylgroups comprising at least one heteroatom. Halo includes fluoro, chloro,bromo, and iodo. Substituted groups may contain one or moresubstituents. Suitable substituents may include, for example, H, C₁-C₂₀alkyl, C₂-C₄₀ alkenyl, C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl, C₃-C₈heterocycloalkyl, aryl, heteroaryl, aralkyl, heteroaralkyl, halo, amino,azido, nitro, carboxyl, cyano, aldehyde, alkylcarbonyl, aminocarbonyl,hydroxyl, alkoxy, and the like. Substituents can also beelectron-withdrawing groups and electron-donating groups.

Metal complexes suitable for exciton blocking layers include those thathave relatively wide optical gaps. Metal complexes suitable for thepreparation of hole blocking layers include high energy absorbers andemitters, such as, for example, blue emitters. Preferred metal complexesinclude those in which the metal has a closed valence shell (no unpairedelectrons). As a result, many preferred metal complexes for preparingexciton blocking layers are colorless, since their optical gap energyfalls outside the visible range. Further, complexes having heavy metalsare preferred. For example, heavy metals of the second and third rowtransition series tend to have larger optical gaps due to a strongerligand field. Examples of suitable metal complexes for exciton blockinglayers include, inter alia, complexes of Os, Ir, Pt, and Au, such asthose described in application Ser. No. 08/980,986, filed Dec. 1, 1997,Ser. No. 09/883,734, filed Jun. 18, 2001, and Ser. No. 60/283,814, filedApr. 13, 2001, each of which is herein incorporated by reference in itsentirety. In some embodiments, metal complexes suitable for excitonblocking layers include FIrpic, Ga(pma)₃, and related compounds. Othersuitable complexes include those described in Example 1.

The HOMO and LUMO energy levels for OLED materials, can be measured, orestimated, in several ways known in the art. The two common methods forestimating HOMO energy levels include solution electrochemistry, such ascyclic voltammetry, and ultraviolet photoelectron spectroscopy (UPS).Two methods for estimating LUMO levels include solution electrochemistryand inverse photoemission spectroscopy. As discussed above, alignment ofthe HOMO and LUMO energy levels of adjacent layers can control thepassage of holes and electrons between the two layers.

Cyclic voltammety is one of the most common methods for determiningoxidation and reduction potentials of compounds. This technique is wellknown to those skilled in the art, and a simple description of thisprocess follows. A test compound is dissolved along with a highconcentration of electrolyte. Electrodes are inserted and the voltagescanned in either the positive or negative direction (depending onwhether an oxidation or reduction is being performed). The presence of aredox reaction is indicated by current flowing through the cell. Thevoltage scan is then reversed and the redox reaction is reversed. Thereference can be an external electrode, such as Ag/AgCl or SCE, or itcan be an internal one, such as ferrocene, which has a known oxidationpotential. The latter is often preferred for organic solvents, since thecommon reference electrodes are water based. A useful parameter that maycome from cyclic voltammetry is the carrier gap. If both the reductionand oxidation are reversible, one can determine the energy differencebetween the hole and the electron (i.e. taking an electron out of theHOMO versus putting one into the LUMO). This value can be used todetermine the LUMO energy from a well defined HOMO energy. This methodfor determining redox potentials and reversiblity of redox events usingcyclic voltammetry is well known in the art.

UPS is an alternative technique for determining absolute bindingenergies in the solid state. Although solution electrochemistry istypically adequate for most compounds, and for giving relative redoxpotentials, the measurements taken in the solution phase can differ fromvalues in the solid phase. A preferred method of estimating HOMOenergies in the solid state is UPS. This is a photoelectric measurement,where the solid is irradiated with UV photons. The energy of the photonsare gradually increased until photogenerated electrons are observed. Theonset of ejected electrons gives the energy of the HOMO. The photons atthat energy have just enough energy to eject an electron from the top ofthe filled levels. UPS provides HOMO energy level values in eV relativeto vacuum which corresponds to the binding energy for the electron.

Inverse photoemission may be used to directly estimate LUMO energylevels. This technique involves pre-reducing the sample and then probingthe filled states to estimate the LUMO energies. More specifically, amaterial is injected with electrons which then decay into unoccupiedstates and emit light. By varying the energy of the incoming electronsand the angle of the incident beam, electronic structure of a materialcan be studied. Methods of measuring LUMO energy levels using inversephotoemission are well known to those skilled in the art.

Optical gap values can be determined from the intersection of thenormalized absorption and emission spectra. For molecules that have verylittle structural rearrangement in going from the ground state to theexcited, such that the gap between the absorption and emission λ_(max)values is rather small, the intersection energy is a good estimate ofthe optical gap (the 0-0 transition energy). Thus, the optical gaproughly corresponds to the HOMO-LUMO gap, and such estimation may beadequate for ideal systems. However, if the shift between the absorptionand emission maxima is large (Stokes shift) the optical gap can be moredifficult to determine. For example, if there is a structuralrearrangement in the excited state or the measured absorption does notrepresent the lowest energy excited state, then there can be asubstantial error. Thus, for the selection of potential exciton blockingmaterials, the edge of the absorption band of the material is preferablyused to obtain a value for its optical gap. In this way, device layerscomprising materials having absorption band energies higher than foradjacent layers may serve as effective exciton blocking layers. Forexample, if an exciton approaches a layer in a device having a higherenergy absorption edge than the material containing the exciton, theprobability that the exciton will be transferred into the higher energymaterial is low. For molecules emitting from triplet excited states, theabsorption edge is a preferred estimate for optical gap, since theintersystem crossing leads to a very large Stokes shift.

Light emitting devices of the present invention can be fabricated by avariety of techniques well known to those skilled in the art. Smallmolecule layers, including those comprised of neutral metal complexes,can be prepared by vacuum deposition, organic vapor phase deposition(OVPD), such as disclosed in U.S. Pat. No. 6,337,102, which isincorporated herein by reference in it its entirety, or solutionprocessing, such as spin coating. Polymeric films can be deposited byspin coating and CVD. Layers of charged compounds, such as salts ofcharged metal complexes, can be prepared by solution methods such a spincoating or by an OVPD method, such as disclosed in U.S. Pat. No.5,554,220, which is incorporated herein by reference in its entirety.Layer deposition generally, though not necessarily, proceeds in thedirection of the anode to the cathode, and the anode typically rests ona substrate. As such, methods of fabricating devices, involvingdepositing a blocking layer that comprises a metal complex onto apreexisting layer, are also encompassed by the present invention.Preexisting layers include any layer that is designed to be in contactwith the blocking layer. In some embodiments, the preexisting layer canbe an emissive layer or a HTL. Devices and techniques for theirfabrication are described throughout the literature and in, for example,U.S. Pat. Nos. 5,703,436; 5,986,401; 6,013,982; 6,097,147; and6,166,489. For devices from which light emission is directedsubstantially out of the bottom of the device (i.e., substrate side), atransparent anode material, such as ITO, may be used as the bottomelectron. Since the top electrode of such a device does not need to betransparent, such a top electrode, which is typically a cathode, may becomprised of a thick and reflective metal layer having a high electricalconductivity. In contrast, for transparent or top-emitting devices, atransparent cathode may be used, such as disclosed in U.S. Pat. Nos.5,703,436 and 5,707,745, each of which is incorporated herein byreference in its entirety. Top-emitting devices may have an opaqueand/or reflective substrate, such that light is produced substantiallyout of the top of the device. Devices can also be fully transparent,emitting from both top and bottom.

Transparent cathodes, such as those used in top-emitting devicespreferably have optical transmission characteristics such that thedevice has an optical transmission of at least about 50%, although loweroptical transmissions can be used. In some embodiments, devices includetransparent cathodes having optical characteristics that permit thedevices to have optical transmissions of at least about 70%, 85%, ormore. Transparent cathodes, such as those described in U.S. Pat. Nos.5,703,436 and 5,707,745, typically comprise a thin layer of metal, suchas Mg:Ag, with a thickness, for example, that is less than about 100 Å.The Mg:Ag layer can be coated with a transparent,electrically-condutive, sputter-deposited, ITO layer. Such cathodes areoften referred to as compound cathodes or as TOLED (transparent-OLED)cathodes. The thickness of the Mg:Ag and ITO layers in compound cathodesmay each be adjusted to produce the desired combination of both highoptical transmission and high electrical conductivity, for example, anelectrical conductivity as reflected by an overall cathode resistivityof about 30 to 100 ohms per square. However, even though such arelatively low resistivity can be acceptable for certain types ofapplications, such a resistivity can still be somewhat too high forpassive matrix array OLED pixels in which the current that powers eachpixel needs to be conducted across the entire array through the narrowstrips of the compound cathode.

Structures of light emitting devices are often referred to by asequential listing of layer materials separated by slashes. For example,a device having an anode layer adjacent to a hole transporting which isadjacent to an emissive layer which is adjacent to an electron blockinglayer which is adjacent to a cathode layer can be written asanode/HTL/EL/ETL/cathode. As such, devices of the present invention caninclude the substructures HTL/EL/HBL, HTL/EBL/EL, HTL/EBL/ETL, andothers. Some preferred structures of the present invention includeanode/HTL/EL/HBL/ETL/cathode and anode/HTL/EBL/EL/ETL/cathode.

Light emitting devices of the present invention can be used in a pixelfor a display. Virtually any type of display can incorporate the presentdevices. Displays can include computer monitors, televisions, personaldigital assistants, printers, instrument panels, bill boards, and thelike. In particular, the present devices can be used in heads-updisplays because they can be substantially transparent when not in use.

As those skilled in the art will appreciate, numerous changes andmodifications can be made to the preferred embodiments of the inventionwithout departing from the spirit of the invention. It is intended thatall such variations fall within the scope of the invention.

Throughout this specification various groupings are employed toconveniently describe constituent variables of compounds and groups ofvarious related moieties. It is specifically intended that eachoccurrence of such groups throughout this specification include everypossible subcombination of the members of the groups, including theindividual members thereof.

It is intended that each of the patents, applications, and printedpublications mentioned in this patent document be hereby incorporated byreference in their entirety.

EXAMPLES Example 1 Iridium Complexes Suitable for Blocking LayersAccording to the Present Invention

The table given below has the HOMO energies, based on UPS, the carriergaps (cyclic voltammetry) and the LUMO energies. Absorption band edgesare also given. All of the complexes have fully reversible oxidation andreduction waves. The structures of the complexes are shown in FIG. 1.

TABLE 1 Properties of Ir complexes LUMO oxididation reduction carrierHOMO (eV, HOMO - Optical cpd (V, vs. Fc/Fc+) (V, vs. Fc/Fc+) gap (V) (eVfrom UPS) carrier gap)) gap (eV) mer- 0.25 −2.5 2.75 5.23 2.48 2.5 Irppyfac- 0.32 −2.69 3.01 5.36 2.35 2.59 Irppy BTPIr 0.36 −2.29 2.65 5.142.49 2.35 PQIr 0.43 −2.45 2.88 5.41 2.53 2.18 BSNIr 0.55 −2.18 2.73 5.242.51 2.32 BTIr 0.56 −2.15 2.71 5.45 2.74 2.17

Example 2 Synthesis of 2-(4,6-difluorophenyl)pyridine

The compound 2-(4,6-difluorophenyl)pyridine was prepared by Suzukicoupling of 4,6-difluoropheyl boronic acid (Frontier Chemical) with2-bromopyridine (Aldrich) in 1,2-dimethoxyethane using a Pd(OAc)₂/PPh₃catalyst and K₂CO₃ base according to Synlett, 1999, 1, 45, which isincorporated herein by reference in its entirety.

Example 3 Synthesis offac-tris(2-(4,6-difluorophenyl)pyridinato-N,C2′)iridium(III)

Ir(acac)₃ was treated with 6 eq of 2-(4,6-difluorophenyl)pyridine inglycerol at 180 C under an inert gas atmosphere for 16 hours. Aftercooling to room temperature, water was added to the reaction mixture inorder to precipitate the crude product. The solvent was removed underreduced pressure and the crude yellow product was washed with methanol.The crude product was flash chromatographed using asilica:dichloromethane column to yield ca. 75% of the pure yellowfac-tris(2-(4,6-difluorophenyl)pyridinato-N,C2′)iridium(III) productafter solvent evaporation and drying.

Example 4 Synthesis of [(2-(4,6-difluorophenyl)pyridyl)₂IrCl]₂

All procedures involving IrCl₃-H₂O or any other Ir(III) species werecarried out in inert gas atmosphere. A mixture of IrCl₃.nH₂O and 4 eq of2-(4,6-difluorophenyl)pyridine in 2-ethoxyethanol was heated at 130° C.for 16 hr. the product was isolated by addition of water followed byfiltration and methanol wash giving 90% yield.

Example 5 Synthesis of bis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III) picolinate (FIrpic)

The complex [(2-(4,6-difluorophenyl)pyridyl)₂IrCl]₂ was treated with 2eq of picolinic acid in refluxing 1,2-dichloroethane under inert gas for16 hours. After cooling to room temperature, the solvent was removedunder reduced pressure and the crude yellow product was washed withmethanol to remove any unreacted picolinic acid. The crude product wasflash chromatographed using a silica:dicloromethane column to yield ca.75% of the pure yellow product after solvent evaporation and drying.

Example 6 Synthesis of Ga(III)tris[2-(((pyrrole-2-yl)methylidene)amino)ethyl]amine (Ga(pma)₃)

The ligand [(((pyrrole-2-yl)methylidene)amino)ethyl]amine was preparedby adding a methanolic solution of pyrrole-2-carboxaldehyde (1.430 g, 15mmol, 100 mL) to a methanolic solution of tris(2-aminoethyl)amine (0.720g, 5 mmol, 10 mL). The resulting yellow solution was stirred at roomtemperature for 30 min. A methanolic solution of gallium(III) nitratehydrate (1.280 g, 5 mmol, 150 mL) was added to the ligand solution andstirred at room temperature for 30 min. The solution was filtered andleft to stand at ambient temperature until crystallization occurred. Thecrude material was then sublimed at 235° C.

Example 7 Light Emitting Devices Having a Blocking Layer Comprising aWide Band-Gap Matrix Doped with a Metal Complex

Devices have been fabricated that comprise hole blocking layers whichconsist of FIrpic doped into a wide gap matrix. These doped layerscomprise less of the Ir complex than a layer made of pure Ir complex andmay have an advantage in long term stability. The details of thisprocess are described below.

The HBL was tested in blue phosphorescent OLEDs with the structureITO/NPD(300 Å)/CBP:FirPic(5%)(300 Å)/HBL/ETL/Mg:Ag/Ag, where ITO isindium-tin oxide, NPD is 4,4′bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl,CBP is 4,4′-N,N′-dicarbazole biphenyl, FIrPic isbis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III) picolinate. As ahole-blocking matrix OPCOT (octaphenyl cyclooctatetraene C₈Ar₈, wideband gap (3.3 eV) material) and hexaphenyl compounds were used in whichFIrPic was introduced as both a blue phosphorescence emitter (in CBPemissive layer) and as a dopant (in OPCOT or hexaphenyl) in the HBL.

It was demonstrated that doping OPCOT with 15% FIrPic greatly enhancesthe electron conduction and electron injection properties of the HBL ascompared with the non-doped OPCOT (by lowering LUMO level energy). Thus,OPCOT:FirPic can be used as an efficient HBL. The efficiency of thedevices were comparable to BCP HBL OLEDs(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, a materialconventionally used as a HBL). Moreover, OPCOT:FirPic is not only anefficient hole-blocker, but is also a very good electron-injectionmaterial, since the lower LUMO level of FIrPic as compared to theworkfunction of magnesium, makes it easy to accept electrons from thecathode.

By choosing an appropriate dopant with certain HOMO and LUMO energieslevels it is possible to tune electron conductivity and other OLEDproperties (turn on voltage, quantum efficiency and etc).

Example 8 Devices According to the Present Invention

The tables below summarize efficiencies and turn on voltages for deviceshaving a hole blocking material comprising OPCOT doped with FIrpic andcomparative devices having traditional hole blocking materials.

TABLE 2 NPD/CBP:FIRPIC/OPCOT/OPCOT:FIRPIC(Alq₃) V at 2 Max Devicestructure CIE Color V turn on Cd/m² q.e. % @V NPD (300Å) 0.14 0.29 Aqua4.0 4.9 0.4 8.0 CBP:FirPic 5% (300Å) OPCOT (300Å) NPD (300Å) 0.14 0.25Aqua 5.3 7.0 0.81 11.4 CBP:FirPic 5% (300Å) OPCOT (300Å) OPCOT:FirPic15% (200Å) NPD (300Å) 0.15 0.34 Aqua- 5.0 6.4 0.61 10.8 CBP:FirPic 5%(300Å) Greenish OPCOT (300Å) Alq₃ (200Å)

TABLE 3 NPD/CBP:FIRPic/HBL/ETL V at 2 Max Device structure CIE Color Vturn on Cd/m² q.e. % @V NPD (300Å) 0.15 0.30 Aqua 5.4 6.0 0.35 10.0CBP:FirPic 5% (300Å) OPCOT:FirPic 15% (300Å) NPD (300Å) 0.18 0.36 Aqua4.3 5.1 0.77 9.1 CBP:FirPic 5% (300Å) OPCOT:FirPic 15% (300Å) Alq₃(200Å) NPD (300Å) 0.14 0.29 Aqua 3.8 4.7 1.0 8.2 CBP:FirPic 5% (300Å)BCP (150Å) Alq₃ (200Å)

TABLE 4 NPD/CBP:Irppy₃/HBL/Alq₃ λ max., V turn V at 100 Max Devicestructure nm Color on Cd/m² q.e. % @V NPD (400Å) 516 Green 3.5 4.0 2.77.0 CBP:Irppy₃ (6%) (200Å) BCP (150Å) Alq₃ (200Å) NPD (400Å) 518 Green2.9 3.7 4.1 5.4 CBP:Irppy₃ (6%) (200Å) FirPic (150Å) Alq₃ (200Å) NPD(400Å) 514 Green 4.0 4.5 1.7 7.0 CBP:Irppy₃ (6%) (200Å) OPCOT:FirPic(15%) (150Å) Alq₃ (200Å)

TABLE 5 NPD/CBP:Firpic/BCP/Alq₃ v. NPD/CBP:Firpic/Firpic/Alq₃ λ max., Vturn V at 2 Max Device structure nm Color on Cd/m² q.e. % @V NPD (400Å)476 Aqua 4.0 4.6 0.68 6.8 CBP:Firpic (300Å) 500 BCP (150Å) Alq₃ (200Å)NPD (200Å) 476 Aqua 4.0 4.2 0.61 7.4 CBP:Firpic (300Å) 500 Firpic (150Å)Alq₃ (200Å)

1. A light emitting device, comprising an emissive layer and at leastone blocking layer, wherein said blocking layer comprises at least onetransition metal complex, and wherein said at least one transition metalcomplex is an organometallic complex having a coordinating carbon. 2.The light emitting device of claim 1, wherein said blocking layer doesnot electroluminesce in said device.
 3. The light emitting device ofclaim 1, wherein said blocking layer is a hole blocking layer.
 4. Thelight emitting device of claim 1, wherein said blocking layer is anexciton blocking layer.
 5. The light emitting device of claim 1, whereinsaid blocking layer consists essentially of said metal complex.
 6. Thelight emitting device of claim 1, wherein said transition metal is asecond or third row transition metal.
 7. The light emitting device ofclaim 1, wherein said transition metal is iridium.
 8. The light emittingdevice of claim 1, wherein said metal complex isbis(2-(4,6-difluorophenyl)pyridyl-N,C2 ′)iridium(III) picolinate.
 9. Alight emitting device, comprising an emissive layer and at least oneblocking layer, wherein said blocking layer comprises at least one metalcomplex, comprising a main group metal atom having an atomic numbergreater than 13, and wherein said at least one metal complex is anorganometallic complex having a coordinating carbon.
 10. The lightemitting device of claim 9, wherein said blocking layer does notelectroluminesce in said device.
 11. The light emitting device of claim9, wherein said blocking layer is a hole blocking layer.
 12. The lightemitting device of claim 9, wherein said blocking layer is an excitonblocking layer.
 13. The light emitting device of claim 9, wherein saidblocking layer consists essentially of said metal complex.
 14. The lightemitting device of claim 9, wherein said main group metal atom is athird, fourth, or fifth main group metal atom.
 15. The light emittingdevice of claim 9, wherein said main group metal atom is gallium.
 16. Alight emitting device, comprising an emissive layer and at least oneblocking layer, wherein said blocking layer comprises at least one metalcomplex comprising a main group metal atom, and wherein said at leastone metal complex is a six-coordinate organometallic complex having acoordinating carbon.
 17. A light emitting device, comprising an emissivelayer and a blocking layer, wherein said blocking layer comprises a wideband-gap organic matrix into which an organometallic complex having ametal-coordinating carbon is doped.
 18. A light emitting device,comprising an emissive layer and a hole blocking layer, each of saidlayers having an anode side and a cathode side, wherein said cathodeside of said emissive layer is in contact with said anode side of saidhole blocking layer, wherein said hole blocking layer has a lower HOMOenergy level than the HOMO energy level of said emissive layer, andcomprises at least one transition metal complex, wherein said at leastone transition metal complex is an organometallic complex having acoordinating carbon.
 19. The light emitting device of claim 18, whereinthe magnitude of the difference between the LUMO energy levels of saidhole blocking layer and said emissive layer is less than the magnitudeof the difference between said HOMO energy levels of said hole blockinglayer and said emissive layer.
 20. The light emitting device of claim18, wherein said hole blocking layer consists essentially of said metalcomplex.
 21. The light emitting device of claim 18, wherein saidemissive layer comprises a host material doped with an emitter.
 22. Thelight emitting device of claim 21, wherein said hole blocking layercomprises a wide band-gap organic matrix doped with said metal complex.23. The light emitting device of claim 22, wherein said metal complexhas a smaller band-gap than said matrix.
 24. The light emitting deviceof claim 23, wherein the LUMO energy level of said metal complex is lessthan about 200 meV from the LUMO energy level of said emissive layer.25. The light emitting device of claim 21, wherein said emitter is ametal complex.
 26. A light emitting device, comprising an emissive layerand a hole blocking layer, each of said layers having an anode side anda cathode side, wherein said cathode side of said emissive layer is incontact with said anode side of said hole blocking layer, wherein saidhole blocking layer has a lower HOMO energy level than the HOMO energylevel of said emissive layer and comprises at least one metal complexcomprising a main group metal atom having an atomic number greater than13, wherein said at least one transition metal complex is anorganometallic complex having a coordinating carbon.
 27. A lightemitting device, comprising an emissive layer and a hole blocking layer,each of said layers having an anode side and a cathode side, whereinsaid cathode side of said emissive layer is in contact with said anodeside of said hole blocking layer, wherein said hole blocking layer has alower HOMO energy level than the HOMO energy level of said emissivelayer and comprises at least one six-coordinate transition metalcomplex, wherein said at least one transition metal complex is anorganometallic complex having a coordinating carbon.
 28. A lightemitting device, comprising an emissive layer and an exciton blockinglayer, wherein said emissive layer is in contact with said excitonblocking layer, wherein said exciton blocking layer has a wider opticalgap than the optical gap of said emissive layer, and wherein saidexciton blocking layer comprises at least one organometallic complexhaving a coordinating carbon.
 29. The light emitting device of claim 28,wherein said exciton blocking layer has a HOMO energy level that is lessthan about 200 meV from the HOMO energy level of said emissive layer.30. The light emitting device of claim 28, wherein said exciton blockinglayer has a LUMO energy level that is less than about 200 meV from theLUMO energy level of said emissive layer.
 31. The light emitting deviceof claim 28, wherein said exciton blocking layer consists essentially ofsaid metal complex.
 32. A light emitting device, having the structureanode/hole transporting layer/emissive layer/hole blockinglayer/electron transporting layer/cathode, wherein said hole blockinglayer comprises a wide band-gap organic matrix doped with anorganometallic complex having a coordinating carbon.
 33. A method ofconfining holes to an emissive layer in a light emitting device, saidmethod comprising providing a device according to claim 18, and applyinga voltage across said device.
 34. A method of confining excitons to aemissive layer in a light emitting device, said method comprisingproviding a device according to claim 28, and applying a voltage acrosssaid device.
 35. The method of claim 34, wherein said blocking layerconsists essentially of said metal complex.
 36. The method of claim 34,wherein said blocking layer comprises a matrix doped with said metalcomplex.
 37. A method of fabricating a light emitting device, saidmethod comprising depositing a blocking layer onto a preexisting layer,wherein said blocking layer comprises at least one transition metalcomplex, and wherein said at least one transition metal complex is anorganometallic complex having a coordinating carbon, and wherein saiddevice is a device according to claim
 1. 38. The method of claim 37,wherein said metal complex is Flrpic.
 39. A pixel, comprising the lightemitting device of claim
 1. 40. A pixel, comprising the light emittingdevice of claim
 9. 41. A pixel, comprising the light emitting device ofclaim
 16. 42. A pixel, comprising the light emitting device of claim 17.43. An electronic display, comprising the light emitting device ofclaim
 1. 44. An electronic display, comprising the light emitting deviceof claim
 9. 45. An electronic display, comprising the light emittingdevice of claim
 16. 46. An electronic display, comprising the lightemitting device of claim
 17. 47. The light emitting device of claim 1,wherein said at least one transition metal complex comprises a nitrogencontaining ligand selected from the group consisting of amines,nitrenes, azide, diazenes, triazenes, nitric oxide,polypyrazolylborates, 2,2′-bipyridine, 1,10-phenanthroline, terpyridinepyridazine, pyrimidine, purine, pyrazine, pyridine, 1,8-napthyridine,pyrazolate and imidazolate.